The following invention relates to a dual channel time domain reflectometer in which two data channels may be synchronously sampled with an extremely high degree of data resolution and accuracy.
Time domain reflectometers (TDR's) are used to measure impedance anomalies in cables and other devices under test. Typcially, a time domain reflectometer comprises a current pulse source which is connected to a cable under test or a cable leading to a device uner test. Another point on the cable is connected to an oscilloscope. The current source provides a pulse to the cable/device under test and any impedance discontinuities or mismatches may be viewed as reflections of the current pulse on an oscilloscope. An example of such a system is shown in Frye, U.S. Pat. No. 3,434,049.
Advances in semiconductor chip technology have created a need for measuring and analyzing integrated circuit devices which respond very quickly and with minimal distortion to input waveforms. Such devices are difficult to test, however, because of the inherent data resolution limitations in the current generation of test equipment. For example, it would frequently be advantageous to know the propagation time of a pulse through an integrated circuit, and also the shape of the output wafeform as a function of a test input. These measurements are now possible in a single channel time domain reflectometer, which can only measure a reflection of an input pulse at either the input or the output of a device. It is possible to tie together two identical time domain reflectometers in order to make differential measurements, or to measure both the reflection of a test pulse at the input of a device under test and the resulting output pulse. But, while in theory such an arrangement can be constructed, as a practical matter it would have little utility due to timing errors which would make it impossible to sample both the input and output of a device under test at the same time. In order to sample and analyze differential inputs or both a reflected input pulse and the resulting output pulse from a device under test, it is necessary to have extremely accurate timing resolution. Moreover, it is necessary to ensure that the test pulse is isolated from the sampling circuit so that internal reflections of the test pulse do not degrade the input waveforms.
In prior art devices such as that shown in FIG. 1, a current pulse generator 1 is connected to a sampling head 2 by means of a transmission line 3. The sampling head 2 is then connected to a device under test (DUT) 4. At the interface between the transmission line 3 leading from the current pulse generator 1 to the sampling head 2, however, multiple reflections of a test pulse could occur which effectively degrade the wave shape of any input pulse to be sampled by the sampling head 2. Moreover, with this type of arrangement, a dual channel TDR having the necessary timing resolution would be difficult to construct because of ambiguities in the timing of pulses transmitted over the internal transmission line.